A blackout is described as a period of unconsciousness or lack of awareness when you are unable to recall what happened or what you did. Blackouts may occur as a result of brain damage, drug side effects, excessive alcohol consumption, or disorders affecting brain function, such as epilepsy. Fainting, also known as syncope, is a term used to refer to a blackout. Conditions that can cause syncope include cardiac arrhythmias (abnormal heart rate or rhythm), abnormalities of the heart muscle or valves, or a condition called postural hypotension, in which a person faints after standing up quickly from a lying position and there is insufficient blood flow to the brain at that point.
Blackouts can also be due a recent traumatic event, in which case you may forget everything that happened right before or right after the event (anterograde amnesia). Unexplained blackouts, or blackouts that appear to be due to injury or trauma, should be evaluated by a neurologist.